1. Field of the Invention
The present invention relates to a valve timing control device and, in particular, to a valve timing control device for controlling an angular phase difference between a crank shaft and a cam shaft of a combustion engine.
2. Description of the Prior Art
In general, valve timing of a combustion engine is determined by valve mechanisms driven by a cam shaft according to a characteristic of the combustion engine or the use of the combustion engine. Since conditions of combustion are changed in response to the rotational speed of the combustion engine, it is difficult to obtain optimum valve timing through the whole rotational range. Therefore, a valve timing control device which is able to change valve timing in response to the condition of the combustion engine has, in recent years, been proposed as an auxiliary mechanism of the valve mechanism.
A conventional device of this kind is disclosed, for example, in U.S. Pat. No. 4,858,572. This device includes a rotor which is fixed on an outer projecting end of a cam shaft rotatably supported on a cylinder, a drive member which is driven by the rotational torque from a crank shaft and which is rotatably mounted on the outer projecting end of the cam shaft so as to surround the rotor, a plurality of chambers which are defined between the drive member and the rotor and each of which has a pair of circumferentially opposed walls and a plurality of vanes which are mounted to the rotor and which are extended outwardly therefrom in the radial direction into the chambers so as to divide each of chambers into a first pressure chamber and a second pressure chamber. In this device, a fluid under pressure is supplied to a predetermined one of either the first pressure chamber or the second pressure chamber in response to the running condition of the combustion engine and an angular phase difference between the crank shaft and the cam shaft is controlled so as to advance or retard the valve timing relative to the crank shaft. The valve timing control device is in the position of the maximum advanced condition when each of the vanes contacts with one of the opposed walls of each of the chambers. On the other hand, the valve timing control device is in the position of the maximum retarded condition when each of the vanes contacts with the other of the opposed walls of each of the chambers.
In the event that the valve timing control device is disposed at the outside of the cylinder head as in the above prior device, if the fluid, which is the same as a fluid for lubricating the engine, leaks from the chambers to outside, the amount of the fluid for operating the valve timing control device is decreased and other devices, which are located in the vicinity of the valve timing control device, become dirty. Therefore, strict fluid-tightness is required for the chambers, and additional seal members are always required for fluid-tight sealing of the chambers. Thereby, the structure of the valve timing control device is complicated.